The vast majority of currently used explosive initiators, i.e., detonators, employ an electric heating wire in contact with an explosive charge. In response to a voltage being applied to the wire, frequently referred to as a bridge wire, current heats the wire to sufficient temperature to cause the explosive charge to be detonated. Representative devices of this type are disclosed in U.S. Pat. Nos. 2,801,585 and 2,878,752.
It has long been recognized that prior art devices of a type disclosed in the '585 and '752 patents are subject to accidental firing due to electrostatic discharge (ESD) or electromagnetic interference (EMI) induced current. The danger of accidental, i.e., premature, firing is particularly acute in situations where electro-explosive initiators are subjected to lightning discharges or electrostatic discharges accumulated by moving machines, e.g. motors and generators, or people, as well as to intense electromagnetic fields as subsist, for example, on naval ships, spacecraft, and aircraft.
In consequence, it has been the general practice to provide protection devices to prevent ESD and EMI induced current initiation of electro-explosive initiators. It is necessary for these protection devices to have no effect on the sensitivity of the electro-explosive initiator to normal firing signals, usually in the form of DC current. Ideally, the protection device does not result in a substantial increase in the cost of the electro-explosive initiator and is not particularly complex.
Many different approaches have been disclosed and used in the prior art to prevent premature activation of electro-explosive initiators. In one technique, a discharge is initiated at a location displaced from the explosive material, such as at the discharge teeth, as taught in U.S. Pat. No. 2,408,125, or at the spark gap, as disclosed in U.S. Pat. Nos. 3,180,263 and 4,261,263. In another approach, a low pass filter including inductive, capacitive, and resistive components, is arranged in various configurations, to prevent radio frequency energy from being coupled to the electro-explosive initiator. Such prior art devices are disclosed, e.g., in U.S. Pat. Nos. 2,821,139 and 4,592,280. Voltage detection devices, including zener diodes, have also been used to activate switches to decouple explosive initiators from ESD and EMI induced currents. Prior art devices including zener diodes for these purposes are disclosed in U.S. Pat. Nos. 4,967,665 and 4,819,560.
The prior art protection devices are relatively expensive and complex, compared to the cost and complexity of the bridge wire, the explosive charge, and the housing for them. A main reason for the high cost and complexity of the prior art devices to protect explosive initiators using heating wires is that the characteristics of the wires, even in the same lot, vary substantially from explosive initiator to explosive initiator. For example, a wire explosive initiator designed to activate an explosive charge in response to a current of 0.6 ampere being applied to it for 1 millisecond may, in fact, activate the charge in response to a current as small as 0.2 amperes being applied to it. The standard deviation, i.e., square root of the variance, of prior art explosive initiators is thus relatively high. Because of this factor, the prior art ESD and EMI induced current protection devices must be designed to tolerate very wide variations in applied current and voltage to prevent premature activation of electro-explosive initiators.
It is, accordingly, an object of the present invention to provide a new and improved protection device for electro-explosive initiators, which protection device is extremely simple and inexpensive.
To achieve this object, the present invention relies on the discovery that the standard deviation of the voltage necessary to activate a recently introduced electro-explosive initiator is extremely low, e.g., 0.05 volts. Because of the low standard deviation of the voltage necessary to initiate an explosive discharge of this device, it is possible to provide adequate protection for both ESD and EMI induced currents by using a very simple voltage detection device that shunts the electro-explosive initiator when the DC voltage applied to the initiator exceeds a predetermined value which is derived from an energizing circuit for the initiator.
The recently-introduced device is disclosed in U.S. Pat. Nos. 4,708,060 and 4,976,200, as well as in a report printed January 1987 entitled "Semiconductor Bridge (SCB) Development Technology Transfer Symposium," Sandia Report, SAND86-2211 (bullet) UC-13, R. W. Bickes, Jr., editor, prepared by Sandia National Laboratories, Albuquerque, N. Mex. 87185 and Livermore, Calif. 94550 for the United States Department of Energy under contract D-AC04-76DP00789. The device includes a non-metallic substrate that carries a highly doped semiconductor (silicon) layer. First and second electrically conductive lands, usually fabricated of aluminum or tungsten, are deposited on the semiconductor layer, such that a gap subsists between them. The substrate may be silicon on which an oxide layer is formed, and the doped Si layer is deposited on the oxide layer. Alternatively, the substrate is sapphire on which is deposited the doped Si layer.
An explosive charge contacts the lands and bridges the gap. In response to a voltage exceeding a predetermined level being applied by the lands across the gap for a predetermined duration, a plasma is established in the gap. The plasma initiates the explosive charge, thereby causing initiation or detonation.
The various layers and electrodes of this prior art device are fabricated using integrated circuit techniques. A non-metallic wafer having, e.g. a diameter of 4-6 inches, is prepared. Masking and deposition techniques are used to form several hundred separate devices on the wafer. After the doped layers and lands are deposited, the wafer is diced into several thousand chips, each including a semiconductor bridge which is mounted on a header and charge holder combination. The resulting structure is then loaded with an explosive charge.
It has been found that electro-explosive initiators manufactured in accordance with these techniques have a very low standard deviation of the voltage necessary to initiate explosive action. Generally, for a particular lot, the standard deviation is about 0.05 volts. This means, for example, that if a particular semiconductor bridge explosive initiator is designed, i.e. rated, to explode in response to a voltage of 5.0 volts being applied to it for 10 microseconds, the likelihood of a DC voltage as low as 4.60 volts initiating an explosive discharge is about one in 100 million! Likewise, a voltage of 5.40 volts will initiate an explosion with a probability of failure of only 1 in 100 million.
It is, accordingly, another object of the present invention to provide an extremely simple and low cost device for preventing both ESD and EMI induced currents from prematurely activating a semiconductor bridge, explosive initiator.